Agents page polish

[morgmart]

What this does

Moves the existing agent-creation button into the Agents page header, labels it Create Agent, and uses the design system’s small button. The existing creation dialog, availability rules, and create/start flow are unchanged.

Work in progress: this PR is the ongoing Agents page polish pass. The header-button update is the first completed change; further polish will be added as it is agreed and implemented. The current changes build on the inventory work in #286 and creation flow in #332 without changing their behavior.

Related: BOT-1820: Agent management

Verification

• Independent source review: no blockers.
• Pre-commit formatting/lint and pre-push TypeScript, 117 related tests, and design-system checks passed at d3b3d9ac.
• All 32 cases in the two affected browser test files passed in Chromium and WebKit on the identical pre-commit source tree. No browser cases added or removed; existing selectors follow the renamed, relocated button.
• Checked header placement, opening the existing dialog, narrow/medium/wide layout, and dark appearance using production components with isolated test data.
• Native desktop and human acceptance remain pending. The browser alone cannot manage real local agents. No real agents were created, started, stopped, or imported during testing.
• Broad validation is left to PR CI. The related unit tests passed with duplicate-key warnings in their repeated-create fixtures.

Screenshots

Production Agents components rendered with synthetic test data, not live-account or native-desktop evidence.

Try in the desktop app

From the feature worktree, coordinate closing other Foundation copies before launching bin/just desktop. Open Agents, check the smaller Create Agent button in the header, and open then cancel the existing dialog without creating an agent. Saved agents may start when the desktop app launches, so this remains an attended check.